The present invention concerns a tire with radial carcass reinforcement and more particularly a tire intended for fitting to vehicles carrying heavy loads and driving at sustained speeds, such as trucks, tractors, trailers or coaches.
A tire of the xe2x80x9cHeavy Dutyxe2x80x9d type generally comprises a radial carcass reinforcement formed of a single ply of metallic reinforcement elements anchored in each bead to at least one bead wire. The said carcass reinforcement is radially covered by a crown reinforcement formed by at least two working plies made of non-extensible metallic reinforcement elements, which are mutually parallel within each ply and crossed over from one ply to the next, making angles which may be between 10xc2x0 and 45xc2x0 with respect to the circumferential direction of the tire. The said working plies are generally completed by a so-termed protection ply made of extensible metallic reinforcement elements, and either by a so-termed triangulation ply of non-extensible metallic elements oriented with respect to the circumferential direction at an angle greater than 45xc2x0 in a known way, or by a ply of reinforcement elements oriented circumferentially, or by both the preceding types of ply.
xe2x80x9cHeavy Dutyxe2x80x9d tires have form ratios H/S, H being the height of the tire on its rim and S being the maximum radial width of the tire when fitted on its working rim and inflated to the recommended pressure, ranging between 0.65 and 1.0. However, xe2x80x9cHeavy Dutyxe2x80x9d tires are now appearing which have smaller form ratios H/S, for example of the order of 0.45.
Whatever the type of tire, it is known that a compromise between the various properties required is difficult to achieve, since unfortunately an improvement in one characteristic usually goes together with a degradation of one or more other properties.
Numerous attempts have been made to try and obtain the best compromise, more particularly in the case of cruising tires. With a view to improving comfort, resistance to punctures and the wear resistance of the tread, the British document GB 359 110 proposes to give the tire, during molding in the vulcanization mold, a shape very similar to its shape under load, while the tread is made circumferentially non-extensible by the presence of an armature of circumferentially continuous cables or wires. Thus, the tire is molded with a low form ratio, markedly incurving sidewalls, and a carcass reinforcement anchored on each side of the equatorial plane to a bead-wire such that at the point of tangency between the meridian profile of the carcass reinforcement and the bead wire arid with respect to a line parallel to the tire""s rotation axis, the common tangent to the bead-wire and the said profile makes an angle which is open axially towards the outside and radially towards the inside.
To achieve a very clear improvement of the compromise between comfort, road-holding and the stability of the tire, U.S. Pat. No. 3,486,547 describes a tire with a low form ratio H/S, which may be between 0.25 and 0.75, and in which the areas of the sidewall close to the edges of the rim are essentially parallel to the rotation axis of the tire, the said areas being reinforced by radially non-extensible rings and the said rings being of various constitutions. Such a structure entails fitting the tire on a rim whose width W is small compared with the maximum axial width S of the said tire, the ratio W/S being between 0.25 and 0.75.
A similar tire is also described in the patent FR 1 267 264, and is intended to enable a compromise between comfort, road-holding, low rolling resistance and high resistance to wear. In order to increase the structural flexibility of the carcass reinforcement assembly considerably while avoiding the disadvantages that automatically go together with such an increase, the said carcass reinforcement has very curved sidewalls and is covered by a cylindrical crown reinforcement which is circumferentially non-extensible and preferably formed of longitudinal reinforcement elements. Near-the anchoring bead-wires, the carcass reinforcement has portions with tangents that are horizontal or situated at radii smaller than the radii of the rim edges, the said portions being reinforced in the case described by circumferential reinforcement elements.
U.S. Pat. No. 4,029,139 also concerns a tire with a form ratio H/S preferably between 0.40 and 0.60 such that the ratio W/S of the rim width W to the maximum axial width S of the tire is smaller than 0.65. It describes a particular system for fitting the beads to the rim, such that the parts of the bead close to the rim are essentially horizontal.
The development of a xe2x80x9cHeavy Dutyxe2x80x9d type tire and the adaptation to the said tire of the principles mentioned above have proved disappointing. In effect, if comfort is improved the general endurance of the tire is not, whether this applies to endurance in terms of wear or the fatigue endurance of the tire""s various reinforcements.
The object of the invention is to improve the general endurance of this type of tire while at the same time improving its rolling resistance.
The tire according to the invention, which has a form ratio H/S between 0.3 and 0.8, comprises a carcass reinforcement of equatorial radius RSSxc2x1xcex94RSS, where xcex94RSS is equal to 1 times the minimum thickness of the said reinforcement, which is covered radially by an essentially or quasi-cylindrical crown reinforcement formed by at least one working ply, of circumferential reinforcement elements, the said carcass reinforcement, on either side of the equatorial plane, being on the one hand tangential to a circle C termed the holding circle, such that the common tangent to the said circle C and to the meridian profile of the carcass reinforcement makes at the tangency point between the said profile and the holding circle, and with respect to a line parallel to the rotation axis passing through the said tangency point, an angle between +20xc2x0 and xe2x88x9280xc2x0, and on the other hand, in its portion between the said tangency point and the point of greatest axial width, being provided with a reinforcement armature made of circumferentially non-extensible elements. It is characterized in that, viewed in meridian section, and when the tire is fitted on its working rim and inflated to the recommended pressure, the centerline of each half of the main portion of the carcass reinforcement has a meridian profile composed of four circular arcs:
a first circular arc TA of radius rxe2x80x2l, comprised between or equal to the quantities r1, and/or 2r1, which is on the one hand tangential to the holding circle C, concentric with the circle of the rim edge of radius r1, and located radially above and separated from the said edge by a distance eT at least equal to twice the minimum thickness e of the carcass reinforcement, and which on the other hand has a point of intersection A with
a second circular arc AE of radius rxe2x80x31, comprised between the quantities r1 and 2r1, which is tangential to the line perpendicular to the rotation axis passing through the point E of greatest axial width,
a third circular arc EF of radius r2, which is tangential to the second arc of a circle AE at the point E of greatest axial width and also tangential to a fourth arc of a circle FG, parallel to the quasi-cylindrical crown reinforcement, at a point F separated from the line parallel to the rotation axis and passing through the center OJ of the holding circle C by a distance d, where the quantities d, r1 and r2 satisfy the relationships:
d=r2+(r1+arJ+eT)cos xcex1 and r1=r2RSS/(RSSxe2x88x92r2)
xe2x80x83where xcex1 is the angle which the tangent at T makes with respect to a line parallel to the rotation axis, and a is a constant which can have the values xe2x88x921,0 or +1, and
a fourth circular arc FG of radius RTC equal to the transverse radius RT of the crown reinforcement, reduced by at most the minimum thickness of the carcass reinforcement,
and in that the width of the working crown reinforcement is comprised between the distance separating the two tangency points F of the said meridian profile to the crown reinforcement and the said distance increased by 2r2/3.
The term xe2x80x98quasi-cylindrical crown reinforcementxe2x80x99 is used to mean a crown reinforcement whose transverse radius of curvature is at least equal to 4 times its equatorial radius of curvature.
The carcass reinforcement is preferably formed of radial reinforcement elements, which may be made of a textile material because of the low stress supported by each reinforcement element, the stress supported being due to the internal inflation pressure and being a function of the meridian profile of the said carcass reinforcement.
It is an advantage for the crown reinforcement to consist on the one hand of an axially continuous ply formed of circumferential non-extensible reinforcement elements, for example metallic cables (elements are termed non-extensible if, under a tensile force equal to 10% of the breaking force, their relative elongation is less than 0.5%), and on the other hand, of the two plies of non-extensible reinforcement elements mutually parallel within each ply and crossed over from one ply to the next, which make an angle between 20xc2x0 and 60xc2x0, and preferably between 40xc2x0 and 55xc2x0 with respect to the circumferential direction.
The term xe2x80x98axial width of the working crown reinforcementxe2x80x99 is used to mean the larger of two widths: the axial width of the crown reinforcement ply formed of circumferential elements and the axial width of the narrower of the crown reinforcement plies formed of inclined elements.
On each side of the equatorial plane and in its portion comprised radially between the tangency point to the holding circle and the point of greatest axial width, the carcass reinforcement is completed by a reinforcement armature which is essentially circumferentially non-extensible, the said armature consisting of a simple bead-wire of known type (braided, xe2x80x9cpackedxe2x80x9d, with rectangular or other wires), or of at least one ply of non-extensible circumferential elements, preferable metallic, as known in its own right. The said reinforcement armature may be located either inside the carcass reinforcement, or outside it, or between its plies, or in all three of the above positions. The said armature is preferably positioned axially around the point of intersection A of the two circular arcs TA and AE.
Preferably, and in a my known in its own right, the axial distance separating the two tangency points of the meridian profile of the carcass reinforcement to the two holding circles is smaller than two-thirds of the largest axial width of the carcass reinforcement.
The tire is mounted on the rim, in a known way, by beads each of which comprises at least one bead-wire around which the carcass reinforcement is anchored by being folded back upon itself. The folded-back portion may be long enough to be considered an integral part of the reinforcement armature located in the lower position of the tire""s sidewall. Due to the meridian profile of the carcass reinforcement, the anchoring bead-wire may have a cross-section smaller than that of the bead-wires currently used in tires of normal shape and having the same axial and radial dimensions. Whenever a carcass reinforcement is folded back upon itself, it is advantageous for the folded-back portion to be located in a zone of low deformation, and it can be lacquered against the main portion of the said reinforcement without the presence of sections between the main and folded-back portions.
The carcass reinforcement can also be held fast in the beads by inserting its edges between two armatures of non-extensible and circumferential reinforcement elements, or between the rim itself and one reinforcement armature as above.
The characteristics and advantages of the invention will be better understood from the description that follows, which refers to the drawings that illustrate exemplary embodiments in a non-limiting way.